1) Field of the Invention
The present invention relates to a method for controlling, in an electrostatic precipitator unit comprising discharge electrodes and collecting electrodes between which a varying high voltage is maintained, a pulsating direct current supplied to the electrodes.
The method is particularly suitable when the pulsating direct current is in the form of a pulse train which is synchronized with the frequency of the voltage from a main source and whose pulses are generated by supplying, by means of a phase angle controlled rectifier (thyristor), part of a half-wave of the mains voltage to the electrodes of the precipitator after step-up transformation, whereupon a plurality of periods of the main voltage may pass without current being supplied to the electrodes. Subsequently, part of a half-wave is again supplied, followed by a plurality of periods without current etc.
2) Description of Related Art
In many contexts, especially in flue gas cleaning, electrostatic precipitators are the most suitable dust collectors. Their design is robust and they are highly reliable. Moreover they are most efficient. Degrees of separation above 99.9% are not unusual. Since, when compared with fabric filters, their operating costs are low and the risk of damage and stoppage owing to functional disorders is considerably smaller, they are a natural choice in many cases.
The requirements of the authorities regarding the level of emissions from e.g. plants in which fossil fuels are combusted, are directed to the total amount of emissions. This means that functional troubles must be taken into consideration. When using electrostatic precipitators, the most frequent trouble is the cleaning of the filter involving rapping, which must be carried out to permit dust deposited on the collecting electrodes to be removed from the filter. In such filter cleaning, the emissions temporarily increase very strongly, if no specific measures are taken. One possible measure is disclosed in EP-162 826.
The total consumption of energy in the electrostatic precipitators in a large incineration plant may amount to several hundred kW. It has therefore become most important to reduce this consumption of energy as far as possible. This is especially important when dust of high resistivity is to be separated. In such cases, it is often necessary to work with extremely unfavorable operational parameters owing to the risk of electric breakdown in the dust layer which successively grows on the collecting electrodes. This leads to charges and dust being emitted from the collecting electrodes, so-called back corona.
In order to optimize the operation and reduce the energy consumption at the same time as the separation is improved, several methods for pulse feeding of the current to the filter have been suggested. Examples are to be found in U.S. Pat. Nos. 4,052,177 and 4,410,849. The former suggests the feeding of pulses in the order of microseconds, which means that the rectifiers become most expensive. The latter suggests pulses in the order of milliseconds, which may be achieved quite simply by selectively controlling ordinary thyristor rectifiers to which main frequency alternating current is supplied.
Independently of the selected technique, one tries of course to use it as efficiently and economically as possible. Above all, the emissions must be lower than the fixed limit values. Next, the costs thereof should be minimized.
The new techniques have resulted in an increasing number of control parameters and, consequently, an increasing complexity in the control systems. Unfortunately, this also means that the actual adjustment may be a major disturbance in the function of the separator. In the same way as the emissions increase during the rapping of the filter, the emissions will increase during the adjustment or during the checking of the control parameters as set.
If adjustment is effected manually by means of the reading on an opacimeter (tester for the optical density of smoke), this takes such a long time that, if the load is frequently changing, the emissions can become so considerable during the actual adjustment that they may certainly become as great an amount of the total emissions as those caused by the filter cleaning operation. Furthermore there is a risk that operational variations affect the adjustment such that the optimization fails if considerable changes in the concentration of dust or gas temperature occur during the time needed for the adjustment.
Moreover, as mentioned above, the actual cleaning of the collecting electrodes by rapping leads to a temporarily strongly increased dust concentration in the emitted gas. Each measurement of the opacity for adjustment of the current supply should therefore be made merely in the periods when no cleaning of the filter is carried out. Since such cleaning takes place frequently in the precipitator which is closest to the combustion chamber, or some other dust source, there is a great risk that the cleaning of the filter still has a decisive negative effect on the adjustment.
Therefore it is extremely important to develop methods for quick and safe adjustment of the current supply to electrostatic precipitators, exclusively based on electrical measurements in the precipitator itself or the associated rectifier. It has proved that even if the cleaning of the filter strongly affects the dust concentration in the gas emitted from the separator, this changes but marginally the relation between current and voltage in a precipitator.
A few experiments with optimization exclusively based on measurement of electric variables have already been made, and U.S. Pat. No. 4,311,491, EP-9090 5714W090 and EP-184 922 may be mentioned as examples. However, these examples suffer from remaining deficiencies in respect of flexibility when modifying the process, and reliability in respect of finding the adjustment that involves a minimum of energy consumption under varying conditions when separating highly resistive dust.
It appears that the methods tried so far do not always result in the optimum combination of parameters when separating highly resistive dust. On the contrary, when changing and apparently deteriorating the combination of parameters, considerable advantages in the form of lower emissions and a lower consumption of energy may be obtained. This is particularly the case for the methods which are based on measurement of the dust concentration, but also for methods suggested to date and based on measurement of electric variables.
Therefore, a main object of the present invention is to provide an improved method for selecting operation parameters for electric precipitators when separating so-called difficult dust, for example highly resistive dust.
A further object of the present invention is to provide a method which, based on the measurement of electric variables only, generally results in a quicker and more reliable adjustment of electrostatic precipitators.